Toxoplasmosis modifies the rat's brain to make them love (and be eaten by) cats. It also infects ~30% of humans, and modifies traits in several bad ways (i.e. schizophrenia and increased reaction time). However, humans are thought to be a "dead end" for the parasite. Thus, there is a significant selection pressure for humans to develop resistance but no (or almost no) pressure for toxoplasmosis to counter human resistance. If so, why are so many people infected?

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Thus, there is a significant selection pressure for humans to develop resistance but no (or almost no) pressure for toxoplasmosis to counter human resistance.

Well, I think in this case "Significant" isn't qualified. Significant selective pressure can come in many forms, but as you mention about 30% of humans are infected with Toxopolasma, and the vast majority remain capable and functional. Remember that selective pressure applies to the ability to take advantage of resources and reproduce. People with toxoplasmosis are almost always capable of surviving and reproducing, so I doubt the pressure is all that much.

There is also significant incentive for Toxoplasma to develop the ability to adapt to humans, as it would open up a whole new resource for them to take advantage of. How fast the parasite is capable of adapting, or even if it's possible to eventually include humans in their life cycle, is something that would take some studying.

Our immune systems, in turn, try to kill the parasite regardless of whether we're being taken advantage of or not. You might be interested in reading up on the Red Queen Hypothesis - a theory that delineates how the immune system is at constant war with its environment and potential new threats.

As for your last question...

... why are so many people infected?

That's easy. :-)

Cats have become a popular pet, and most of the time the parasite doesn't significantly affect us. If either of those changes, you'll see a change in infection rates.